1. Field of the Invention
The present invention relates to a data communication system and, more particularly, to a system having its multiplicity of terminal units connected to a common transmission line so that data may be sent and received in a time division manner between those terminal units.
2. Description of the Prior Art
In recent years, there is being highlighted an office automation system (which will be simply referred to as an "OA") which is intended to improve the efficiency of office operations and tasks. The main direction of the OA of the prior art has been in the individual and typical works executed by the batch processing system. From now on, however, it is desired to automate the office operations on a higher grade, involving so-called "data processing", such as an electronic file, electronic mail or document edition.
On the other hand the possibility of applying high-speed and low-price digital transmission to a familiar network is enhanced with the fast development in the optical communication technique which is based upon the use of optical fibers or light-emitting diodes.
In view of the technical background thus far described, attempts are being made to realize an integrated network system in which a facsimile, a telephone, a word processor, a personal computer or a variety of data terminal units are connected to a common signal transmission line so that data communication can be freely effected between the terminals.
In order to realize such a network, however, the following problems have to be solved.
That is to say, it is necessary to solve the problem which is caused by the fact that many kinds of terminal units are connected to a common transmission line so that the speeds of the data to be handled by the respective terminal units are extremely different. Of the existing terminal units, there are many kinds of different terminals which handle data from a super-low speed of about 50 bps to a higher speed than 1 Mbps. In recent developments in telephone switching, there has been developed a time-division switching device which is made operative to convert an analog signal within a voice range of 0.3 to 3.4 KHz into digital information of 8 KHz (or having a period of 125 us) and of 8 bits (which may be 7 bits) thereby to effect the switching operations at a digital information level. In this case, it is necessary to cope with the data speed of 64 Kbps (i.e., 8 bits.times.8 KHz).
The realization of a data communication system which is effective for coping with many different data speeds is a remarkably important subject of the integrated network of that kind.
A second problem is that, of the terminal units existing at present, there are units for data communication by means of a line switching system and units for data communication by means of a packet switching system so that their respective switching networks exist in an independent manner.
It is, therefore, necessary to connect many kinds of terminal equipments which are so constructed as to effect data communication by those different types of switching systems, with a common transmission line so that the data communications may be performed by either of the two switching systems with a view to ensuring effective communications between the terminals.